The efficacy of alternate sugars, trehalose and lactulose, as a postharvest treatment inProtea‘Sylvia’, to alleviate leaf blackening

Abstract

Most commercially important Protea species and cultivars are susceptible to leaf blackening, a postharvest disorder, in which the foliage develops a brown to black discolouration. Not only does leaf blackening have a significant effect on the quality, it also results in consignment rejection in the case where these flowers are transported to export markets. Currently, the only commercially viable and effective way to delay leaf blackening in some cultivars is to pulse the stems with a glucose solution for a recommended period and/or volume, as soon as possible after harvest. Glucose pulsing deters the inflorescence from drawing carbohydrates from the leaves. In addition, it also acts as an osmolyte by promoting the solution uptake by the flower following cold storage or shipping of dry stems, thereby assisting in maintaining quality and extending vase life. However, glucose pulsing may be difficult to execute when applied on a commercial scale, especially when different species, each with their own glucose concentration and exposure time preferences, are harvested at the same time. Also, some cultivars do not respond to glucose pulsing or they may suffer readily from glucose toxicity. The aim of this study was therefore to assess the efficacy of alternate sugars, trehalose and lactulose, in alleviating leaf blackening. Both sugars have previously been reported to suppress water loss, delay programmed cell death events and delay symptoms of senescence in other cut flowers, but its efficacy to control leaf blackening in Protea has not been established. We tested the effect of these sugars with Protea cultivar 'Sylvia', over a period of 12 days. We found that trehalose has the potential to reduce leaf blackening and maintain flower quality. The effect of glucose is however still more significant than lactulose.